It looks outdated and it got replaced by Breeze a long time ago.
Yeah, I don't think it makes sense to continue shipping non-default 1st-party themes. We moved the color schemes to GHNS and nothing exploded.
FWIW Kwin needs a window decoration when breeze is not available, which is Plastik at the moment
Oh, I guess we can't do this, then. :/
What if a Plastik got hidden from settings?
That's unrelated to this bug report.